1.8 KiB
Metatable Type Syntax
Summary
Add unique syntax to allow for specifying the metatable of a type.
Motivation
There is currently no expression for metatables within the type system, with the exception of hacks using typeof. That isn't great, because metatables are a commonly required feature for things like object-oriented programming. typeof is boilerplate which can likely be eliminated by allowing the expression of metatables through special syntax.
Design
Introduce a context-sensitive metatable keyword which specifies that a table has a metatable.
local class = {}
type Identity = {
field: string,
metatable typeof(class)
}
This could cause confusion with table properties named metatable. To combat this, a lint could be added which will warn upon setting table properties named metatable. To silence this lint, ["metatable"] would need to be used instead.
Alternatives
type T = { metatable {} } could be too similar to field syntax. We could change the keyword to be @metatable, similar to how it is currently expressed as when converting a type to a string. However, this introduces a sigil, which increases cognitive load and harms readability.
We could instead rely on metatable type functions to achieve what this RFC proposes. However, relying purely on setmetatable for metatable type expression isn't ideal because it involves more verbose syntax than what this RFC proposes.
The final alternative is also to just do nothing, and live with the boilerplate code required by the typeof solution. This doesn't add any new functionality.
Drawbacks
Unique syntax always has the downside of an additional learning curve to the language. Context-sensitive keywords and sigils increase cognitive load when trying to read Luau, which isn't great.